linux-kernel - Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

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Message-ID: <CAEXW_YRk9ccJuuC95bv0rg2xBT=P8mYR2tBdot2KUFFfChZ4ow@mail.gmail.com>
Date:   Wed, 21 Dec 2022 18:18:44 +0000
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC 0/2] srcu: Remove pre-flip memory barrier

On Wed, Dec 21, 2022 at 5:20 PM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> On 2022-12-21 07:11, Frederic Weisbecker wrote:
> > On Tue, Dec 20, 2022 at 10:43:25PM -0500, Mathieu Desnoyers wrote:
> >> On 2022-12-20 19:58, Frederic Weisbecker wrote:
> >>> On Wed, Dec 21, 2022 at 01:49:57AM +0100, Frederic Weisbecker wrote:
> >>>> On Tue, Dec 20, 2022 at 07:15:00PM -0500, Joel Fernandes wrote:
> >>>>> On Tue, Dec 20, 2022 at 5:45 PM Frederic Weisbecker <frederic@...nel.org> wrote:
> >>>>> Agreed about (1).
> >>>>>
> >>>>>> _ In (2), E pairs with the address-dependency between idx and lock_count.
> >>>>>
> >>>>> But that is not the only reason. If that was the only reason for (2),
> >>>>> then there is an smp_mb() just before the next-scan post-flip before
> >>>>> the lock counts are read.
> >>>>
> >>>> The post-flip barrier makes sure the new idx is visible on the next READER's
> >>>> turn, but it doesn't protect against the fact that "READ idx then WRITE lock[idx]"
> >>>> may appear unordered from the update side POV if there is no barrier between the
> >>>> scan and the flip.
> >>>>
> >>>> If you remove the smp_mb() from the litmus test I sent, things explode.
> >>>
> >>> Or rather, look at it the other way, if there is no barrier between the lock
> >>> scan and the index flip (E), then the index flip can appear to be written before the
> >>> lock is read. Which means you may start activating the index before you finish
> >>> reading it (at least it appears that way from the readers pont of view).
> >>
> >> Considering that you can have pre-existing readers from arbitrary index
> >> appearing anywhere in the grace period (because a reader can fetch the
> >> index and be preempted for an arbitrary amount of time before incrementing
> >> the lock count), the grace period algorithm needs to deal with the fact that
> >> a newcoming reader can appear in a given index either before or after the
> >> flip.
> >
> > True but the number of preempted tasks is bound and there is a forward progress guarantee.
> >
> >> I don't see how flipping the index before or after loading the unlock/lock
> >> values would break anything (except for unlikely counter overflow situations
> >> as previously discussed).
> >
> > Forward progress guarantee.
>
> Considering a coherent cache, the store-buffer will ensure that the
> index flip eventually reaches all readers. This bounds the time during
> which readers can flood the current index, and therefore guarantees
> forward progress. AFAIK the Linux kernel does not support architectures
> with incoherent caches.
>
> So I still don't see how having the barrier before or after the index
> flip is useful for forward progress.

Even though eventually the writes on either side will make it, I think
you have little lost opportunity without the "D" memory barrier
without satisfying the store-buffer pattern. i.e. even if the idx
update was seen by the read-side, its unlock count contributions to
the now-inactive idx may not be seen.

I think that makes it slightly better for making the grace period end
sooner. I think that's what you "scan both" idea also tries to
achieve, so for that reason you should like "D" as well.

But yes it depends on how forward-progress is defined. If defined as
starvation - no the post/pref flip MBs are not helpful for that
(eventually the writes will make it). If defined as not losing an
opportunity to end the GP sooner, yes the flip-related memory barriers
does help that.

As for whether or not we need "E", that is up for debate. I am
intrigued by Frederick's litmus tests showing that it helps, because
my litmus tests don't show it.

Thanks.